The aromatic diols obtained in accordance with this invention are important compounds for the preparation of polyesters and, in particular, of liquid crystalline polyesters when the two hydroxyl groups are in parallel or coaxial position. The use of phenyl hydroquinone in the synthesis of liquid crystalline polymers is described in U.S. Pat. No. 4,159,365, 4,360,658 and 4,600,765, whereas the use of phenyl biphenol is described in Italian patent application 22746 A/87.
Phenyl substituted aromatic diols can be prepared from the corresponding quinones by arylation with a diazonium salt of aniline (Journal of Organic Chemistry page 4071, 1977). Such a process generally presents considerable drawbacks from a practical point of view because the starting products can not always be easily found on the market. Moreover, aniline presents a potential hazard as a carcinogen.
Catalytic dehydrogenation is described in U.S. Pat. No. 4,798,911. In this patent, a catalyst composition is claimed, based on the combination of Pd and Cu, which is loaded on a carbon support. The catalyst is prepared by contacting a heat-treated carbon support with a Pd-containing compound and a Cu-containing compound in a solvent. The catalyst is calcinated prior to use under conditions effective to decompose the Pd- and Cu-containing compounds employed for catalyst preparation. In particular the dehydrogenation of cyclohexylhydroquinone is set forth in Table 3 with high conversion (88%) and high selectivity (90%). By comparison in the same Table, it is shown that a similar catalyst containing 5% Pd and no Cu on carbon gives 100% conversion but extremely low selectivity (4%). In Table 3 the results of Pd-Cu/Al.sub.2 O.sub.3 and Pd-Cu/SiO.sub.2 catalysts are also set forth. Conversion is however highly unsatisfactory.